Upvote:1
Yes, you can. The Schengen area implies free travel of persons. That means, your passport will not be checked by immigration (you still have to identify yourself to the airline for which you may have to use your passport, but that's the airline, not a government official who can deny you access to the country). If you travel Tehran - Leipzig - Bilbao - Leipzig - Tehran, the only place immigration checks your passport is Leipzig (both on arrival and departure). Hence, your visa is only checked once, in Leipzig.
Upvote:3
Multiple entry Schengen visa is usually issued for a long period - several months or a whole year. If your trip will last several days (a week or so) than single entry visa is enough with one condition - you must not leave the territory of Schengen zone. If you want to use planes during your visit your passport is needed only to check you personality.